One known way of making fiber preforms for parts made of composite material consists in superposing layers of fiber fabric and in needling the layers together in order to bind them together in a direction that extends transversely to the layers. Such a method is described, for example, in Document FR-A-2 584 106, in which the layers are needled as and when they are superposed.
The layers are constituted by a fiber fabric that is generally two dimensional, e.g. a cloth or a felt. More complex fabrics may be used, e.g. sheets of cables extending in a single direction and pre-needled together or held together by a fiber web, or a cloth or a web of pre-needled fibers.
This prior technique encounters difficulties of implementation when the thickness of the preform to be made is not constant, with the shape of the preform corresponding to the shape of the part that is to be manufactured.
Under such circumstances, with a preform having a hollow portion of varying thickness, for example, cutouts may be formed in the layers in order to form the desired hollow by superposing the pre-cutout layers.
Unfortunately, because of the non-uniform thickness of the superposed cutout layers, needling is difficult and it becomes practically impossible to perform needling with constant effectiveness within the preform.
An object of the invention is thus to provide a method of making a fiber preform of varying thickness by needling together superposed layers, while enabling the desired shape to be obtained accurately.